In a complete about-face, Florida Sen. Marco Rubio officially announced an eleventh-hour reelection bid Wednesday, making him the instant front-runner in a key battleground state and lifting Republican hopes that they can hang on to the U.S. Senate.

The first-term senator told POLITICO he knows he faces a tough race in a state with a history of close elections and where opponents on the right and left accuse him of rank political opportunism for breaking his pledge to retire from the Senate if he didn’t win his presidential race this year.

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Rubio said the election is too important to miss since the GOP could potentially lose the Senate majority. And in a sign he recognizes the damage his party’s de facto presidential nominee could do to his bid, Rubio said he wouldn’t hesitate to criticize Donald Trump when he makes inflammatory statements or advocates ideas he disagrees with.

“It’s been well-documented that I have significant disagreements with Donald Trump on his failure to articulate policies and many of the things that he has said, especially about women and minorities,” Rubio said in the interview, mentioning Trump’s name unprompted. “And so, I’m prepared to be a senator that will encourage him to make the right decisions, but also stand up to the bad decisions and the bad policies if he’s elected president.”

Rubio didn’t shy away from knocking Trump for drawing attention to the Mexican heritage of an Indiana-born judge who’s presiding over a case against Trump University.

“All I can tell you is that they’re not comments that I agree with. They went beyond the pale. I think that that judge is fully American [and his] experience is not unlike mine,” Rubio, the son of Cuban immigrants, said when asked about Trump’s comments.

Rubio made clear, though, that he believes Hillary Clinton would be worse than Trump: “He’s not running against George Washington.”

Rubio has had a fraught relationship with Trump, who thrashed him so badly in Florida’s March 15 presidential primary that he withdrew from the race. Rubio apologized for making suggestive comments about the size of Trump’s genitalia on the campaign trail and followed through with his commitment to back Trump, who urged Rubio to run for reelection.

But Trump’s campaign has sputtered as his poll numbers have begun to drop in Florida, where Republicans fear he’ll drag down other candidates. Rubio, they say, might be the only candidate in their party with the name ID and rhetorical wherewithal to withstand down-ballot problems if Trump collapses in November.

Still, it’s a delicate balancing act for Rubio, who doesn’t want to estrange the legions of loyal Trump voters who voted against the senator in the primary.

A Quinnipiac University poll released this week shows that Florida’s general-election voters view the two Republicans in vastly different ways.

Trump, once virtually tied with Clinton, is now trailing her by 8 points in the poll. Rubio, though, leads his Democratic opponents, House members Patrick Murphy and Alan Grayson, by 7 and 8 points, respectively.

Still, Democrats tried to see the glass as half full. Presidential election years tend to bring out more Democratic voters, and Trump has been reeling. Democrats believe if they can hand Rubio his second defeat in a year, it would make him damaged goods and potentially close off a 2020 White House bid.

Rubio was characteristically confident, though. He dismissed Murphy, the favorite of national Democrats, as a spoiled rich kid.

“He has no significant achievements in his entire life,” Rubio said. “He has been handed everything, from the moment he was born: his first job, his second job. And now his family and his father want to buy him a Senate seat.”

As for Grayson, Rubio singled out a House ethics investigation and said, “The chances are, he would probably be indicted in his first couple years in office.”

Murphy and Grayson have attacked Rubio for his positions on gay rights and guns, and for breaking his pledge not to run for reelection. Before facing either of them, Rubio still has to win his Aug. 30 primary against political newcomer Carlos Beruff, who bashed Rubio as the choice of Washington power brokers.

Rubio hit back in the interview by pointing out that Beruff had backed his 2010 Senate-race opponent, Gov. Charlie Crist, after the then-governor left the GOP.

“He’ll have to answer for that,” Rubio said.

Beruff said Rubio needs to explain why he broke his word and whether he’ll commit to a full six-year Senate term.

Asked whether he’d run for president in four years, Rubio said: “You know what? I’m done. I’m done making statements like that one way or another.”

Rubio acknowledged that the race for him was made even more difficult because he decided at the last minute and he has to explain to voters why he changed his mind.

“I’m aware of the political arguments of this. You’re running for reelection when you said you wouldn’t in the most unusual national political cycle in modern American history, in the most competitive state in the country, against a well-funded Democrat and other Republicans saying they’re going to run,” he said.

Rubio seriously began entertaining a reelection bid after the Orlando shootings on June 12, when his friend and favored successor, Lt. Gov. Carlos Lopez-Cantera, said the senator should give it a second look. Lopez-Cantera was struggling with fundraising and was stagnant in the polls. He dropped out of the race with Rubio’s entry and praised the senator. U.S. Rep. Ron DeSantis also quit the Senate race Wednesday and announced his reelection bid for the 6th Congressional District. Businessman Todd Wilcox said he’s staying in the race along with Beruff.

Rubio pushed back against the notion that he’s running for reelection because he wants to run for president. His plan to retire — and make a small fortune in the private sector — was geared around running for president as early as 2020. By being in the Senate, Rubio and his aides say, he knows he’ll have to take votes or miss votes that will be held against him if he runs for president again.

Of course, it will also give him a bigger platform and media spotlight than he’d have in South Florida.

Lopez-Cantera disputed the talk of a secret long-term political deal between the two. He said he actually felt guilty because he put the “burden” on Rubio to run for the seat.

“This was a selfless act,” Lopez-Cantera said. “He could have started focusing on his family and their future well-being.”

Rather than plotting out a Senate bid, Rubio said, he spent weekends over the past two months planning to become the defensive coach for his son’s sixth-grade football team and “designing this, if I do say so myself, pretty good defense.

“I was going to get to coach my son directly because he plays safety. I was really excited about that,” Rubio added. “I know that’s kind of a small thing for people outside of it. But for me, that was a pretty big deal. My son will never be in sixth grade again.”

In contrast, there was no Senate campaign to speak of. Only in recent days did Rubio begin reaching out to donors (he might need as much as $50 million) and his departed presidential campaign staffers to see whether they were ready to help out.

Clint Reed, Iowa director for Rubio’s presidential campaign, will be his campaign manager, and Rubio’s longtime consultant, Todd Harris, will serve as a strategic adviser. Rubio’s presidential campaign spokesman, Alex Conant, and campaign manager Terry Sullivan could work for the campaign or with a super PAC. Conservative Solutions, the super PAC formed to help Rubio’s presidential bid, was in the process of winding down when Rubio began considering his reelection.

The senator’s decision to run was widely hailed by Washington and Florida GOP elites who see him as their best hope to keep his seat. They had lobbied Rubio for months to reconsider his pledge to retire in January if he lost his White House bid.

Top Republican leaders said that Rubio’s entry into the race upends the Senate map and may force Democrats to commit far more money to the contest. National Republican Senatorial Committee Chairman Roger Wicker of Mississippi said Rubio will “have a lot of support” from the party and Florida donors.

The decision was so important to Republicans that even notoriously tight-lipped Senate Majority Leader Mitch McConnell held an impromptu gaggle to celebrate. McConnell said Rubio’s decision moved the seat from a “likely loss to an almost certain” hold.

“It has a huge impact on our ability to hold the majority, so it’s a big development,” McConnell said. “I’m just glad Marco had a chance to reassess the job he currently had. … I’m pretty excited about it.”

But Rubio’s fellow Florida senator, Democrat Bill Nelson, indicated that Wicker doesn’t realize how toxic Trump will be in the general election.

Nelson said he and Rubio “get along real well,” but “I think that his biggest problem is that Hillary is going to swamp the top of the ticket and that will have a down-ballot effect. I think Hillary is going to sweep Florida so big that it’s gonna affect all the way down the ballot.”

Rubio said he’s keenly aware of the risks of running for reelection and losing twice in a year — especially in an unpredictable election season like this.

“I understand this was going to be a difficult race whether I announced two weeks ago or two days before qualifying,” Rubio said. “It’s Florida. General election. Very unusual top of the ticket situation for both parties. It’s going to be tough either way.”